ABUJA (Reuters) - Nigeria's ruling party looked set to see its parliamentary majority weaken as results trickled in on Sunday from an election Africa's most populous nation hopes will be its first credible vote in almost two decades.

Election officials and party agents tallied results from 120,000 polling units stretching from the oil-producing mangrove swamps and teeming cities near the southern coast to the dustblown fringes of the Sahara desert in the north.

There were isolated reports of ballot box snatching, clashes between rival supporters in parts of the Niger Delta and two bombs in the remote northeast during the vote but observers said it appeared to have been a vast improvement on previous polls.

"I think it is fair to say this was a real election. It was a real vote," Kenneth Wollack, president of the National Democratic Institute (NDI) which was among foreign groups monitoring the polls, told Reuters.

President Goodluck Jonathan's ruling People's Democratic Party (PDP) is expected to see its parliamentary majority weaken, coming under strong pressure from the opposition Action Congress of Nigeria (ACN) in the southwest and the Congress for Progressive Change (CPC) in parts of the north.

"The ACN is leading in the Senatorial and House of Representatives race in most states in the southwest," said Chinedu Michael, an observer from the Nigerian Committee on the Defence of Human Rights in the commercial capital Lagos.

Media in the southwestern state of Ogun said the speaker of the House of Representatives, Dimeji Bankole of the PDP, lost his seat, while the daughter of former president and PDP stalwart Olusegun Obasanjo lost her bid to remain in the Senate.

At a collation centre in Karo on the outskirts of Abuja, it was clear the CPC was edging out the PDP across most wards being collated there as officials called out numbers above the hum of generators.

"There is no instance of trying to rig, that's why the other parties are moving in," said ruling party representative Theodore Ochei, despite disappointment that the PDP had lost in his ward at the centre of Abuja.

"Even if we lost as a party, we as politicians have more respect. Our leaders might sit up and begin to respect people's views," he said.

LITMUS TEST

But the ruling party gained ground in other areas.

It won seven of 10 House of Representatives seats counted so far in the northern state of Kano, where it had previously held only five. It was also ahead in parts of Borno state in the remote northeast.

Saturday's election, delayed by a week because of widespread logistical glitches, was the first in a cycle which includes presidential elections on April 16 and governorship polls in the country's 36 states on April 26.

It is seen as a litmus test of whether electoral officials can organise a smooth vote and make a break with a long history of polls discredited by ballot stuffing and thuggery.

The African giant has failed to hold a single vote deemed credible by observers since the end of military rule in 1999.

Most Nigerians say the closest the country came to a free election was in 1993, polls which were annulled by former military ruler Ibrahim Babangida, helping pave the way for another six years of military rule.

But Nigerians are hopeful this time will be different.

"I am not tired because I am happy," said Nancy Godwin Bulus, a local observer from a women's group who had sat up through the night for the count in Karo.

"Nigerians are heading for democracy. We are advancing to maturity. We are orderly."